This invention relates generally to detection apparatus and more particularly to detectors of ionizing radiation, such as x-ray and gamma radiation. The invention is described in terms of apparatus useful in x-ray computerized axial tomography systems.
A well known approach for the detection of radiation, such as x-ray and the like, is to provide a pressurized gas filled chamber in the path of the radiation transmission. Within the gas chamber, there are provided a plurality of spaced electrodes which are supported on their opposite ends by ceramic insulating members which are in turn secured to a metal support structure. The spaced electrodes include alternate bias and collector electrode plates, the bias plates being connected to a voltage source and the collector plates being connected to instrumentation for measuring the currents in the individual collector plates. In operation, the radiation enters the gas filled gaps between the electrodes and causes the gas to ionize so as to create photoelectrons and/or ions. These are then biased by the bias electrodes and are collected on the collector electrodes to thereby generate electrical signals which correspond to the degree of ionization in the respective gas filled gaps. In this manner, the degree of radiation within each gap is determined. Such an apparatus is shown and described in U.S. Pat. No. 4,119,853, issued to Shelly et al. on Oct. 10, 1978 and assigned to the assignee on the present invention.
Radiation detectors in general, and especially detectors used in computerized axial tomography, must detect x-ray protons efficiently and with a high degree of resolution. To obtain good spatial resolution, it is desirable to have the electrode plates spaced closely and uniformly over the entire length of the detector. It is also important for each cell, as defined by a pair of adjacent electrode plates, to have identical and stable detecting characteristics. A further complication is that of the likelihood of undesirable microphonics which may exit in such an apparatus. In such a structure where thin metal electrodes must operate in close proximity with a relatively larger electrical potential between them, mechanical vibrations transmitted to the plates may significantly vary the distance between them and thus introduce microphonic current changes which in turn may cause errors in the x-ray intensity measurements. In view of these sensitive structural requirements, the particular fabrication techniques employed have been of utmost importance in obtaining a detector structure with the desired performance characteristics.
The present methods of detector assembly call for the use of a pair of insulator members to extend substantially the length of their respective support members. First, there are formed on one side of each of the insulator members, a plurality of grooves for receiving the electrodes. Then the insulator members are bonded along their entire lengths to their respective metal support members. The support members are then interconnected at the ends and the electrodes are installed and bonded in their respective grooves. Finally, the leads are attached to the electrode plates with alternate leads going to the voltage source and current measuring instrumentation.
Because of its structural stability and insulating characteristics, it has been found that the ceramic material is preferred for use as the insulating members. However, it is recognized that such a material is very brittle and susceptible to breakage if stress concentrations occur. Accordingly, the bonding method by which the ceramic members are adhesively attached to the metal support structures has been found to be most effective. However, it will be seen that such a method is of a relatively permanent nature and does not allow for subsequent disassembly and reassembly of the structure. In other words, if after the assembly is completed, it is found that there are portions that are defective, the entire detector assembly must be discarded since the present method requires the insulating members to be somewhat permanently assembled into the support structure before the electrode plates are installed.
A detector array which is used in computerized tomography systems has an inherent operational characteristic which is cause for furthering the scrappage and replacement costs. By the very nature of its operation, an annular detector array used in a typical rotating detector computerized tomography system is most reliant on that portion of the detector array in the circumferentially central region. This central area of the detector sees most of the body reconstruction area, and the central cell sees the total body reconstruction area. As you proceed outwardly, left or right, from the center cell, the cells see less and less of the total body reconstruction area, and, therefore, contain less total body information. Therefore, the performance specification of an x-ray detector array is generally established such that the standards of performance are hgher for those portions of the array which are in the circumferentially central area thereof. In other words, the specifications generally allow decreasing performance characteristics towards the circumferential ends of the annular detector array but require a very high performance level in the central portion thereof. It will thus be seen that, since performance tests cannot be conducted until the entire array can be assembled, if the specifications are then not met, the entire assembly must be scrapped. Of course, the greatest cause of scrappage is because of the array central portion not meeting specifications. This is true even though those arrays scrapped may have non-central portions which meet the most stringent performance standards. It is, therefore, an object of the present invention to provide an improved radiation detector assembly.
Another object of the present invention is the provision for reducing the amount of scrappage in the production of x-ray detector assemblies.
Still another object of the present invention is the provision for a radiation detector assembly having better performance characteristics in its central portion than in its non-central portions.
Still another object of the present invention is the provision for the conduct of certain performance tests prior to final application of a radiation detector assembly.
These objects and other features and advantages become readily apparent upon reference to the following description when taken in conjunction with the appended drawings.